This invention relates to prostheses mounted upon anchors which are implanted in bone, and for use in dental surgery.
For some years, it has been known to implant in the bone, an anchor to which may be attached a dental prosthesis, which latter may be a single crown or a set of crowns may be secured to a plurality of such implanted anchors. This technique has become practical firstly, because of the realisation that titanium, or high titanium alloy, is a suitable material to employ for the anchor, being non-reactive with bone and other bodY tissue or fluid; and second-y, and more importantly, because an efficacious surgical technique has been developed during recent years for implanting such anchors.
In particular, efficacious surgical techniques of implantation and prosthesis have been developed which have recently been described in--Tissue Integrated prosthesis; Osseo-integration in Clinical Dentistry--Edited by Per-Ingvar Branemark--Quintessence publishing Co Inc. Chicago, 1985.
These techniques have as salient characteristics (a) that the implant is made with such care and precision as to ensure that trauma to the bone is minimised; and (b) that after initial placing of the implant, the implant is left relatively undisturbed for a period of about six months before it is subjected to loading, and in particular before any prosthesis is attached to it During this "unloaded" period, healing and repair of the bone takes place and also the bone will, to a significant extent, integrate with the implanted anchor, the latter being suitably shaped such as to augment the structural integrity of the anchorage when such integration with the bone is accomplished
Various types of titanium anchor are in use. These in general, comprise small plugs which although of sometimes quite complex shape in detail, are generally cylindrical, ranging in length from say 7 to 20 mm and ranging in diameter from say 3 to 7 mm. Some of these anchors have, over all or part of their outer surface, annular ribs or screw threads which may be self tapping threads. Some anchors have, at their foot ends, hollow cores and there may be also transverse through holes; it being intended that during osseo integration during the healing period after initial implantation, bone should reform and move into the hollow cavities of the anchor.
In U.S. Pat. No. 3,499,222, published 10th Mar. 1970, there are described examples of known implant anchors, which are generally cylindrical; which may be externally threaded; and which may have hollow cores and through holes in the walls defining these cores. In U.S. Pat. No. 4,180,910, published 1st Jan. 1980, further anchors for dental implant are described. These are of titanium and are of the kind with apertured hollow cores shaped and intended for re-entry of the bone into the hollow cavities of the anchor during the healing period after initial implantation.
The prosthesis is secured to these anchors by small screw threaded bolts, and usually a transmucosal spacer member is interposed between the prosthesis and the anchor, it being generally the case that the head of the anchor is level with the periphery of the bone, and hence beneath the level of the surface of the mucosa or gum tissue of the patient.
Such a transmucosal spacer element may be secured to the anchor by a small screw-threaded bolt engaging in a preformed threaded axial socket in the head end of the anchor, and, in turn, the prosthesis may be secured to the transmucosal spacer member, it may be by a small screw threaded bolt or in some cases, by a press-fit ball and socket connection which may be cemented by an adhesive.
With an anchor of the type heretofore known, in addition to having at its head end an axial hole threaded to receive fixture bolts, the head end of the anchor may be appropriately shaped so that it can be engaged by a spanner or key. For example it may be provided with outwardly facing spanner flats for receiving an open ended spanner; or it may have a hexagonal socket to receive a small Allen type key. When such a hexagonal socket is provided, it will be located at the outer extremity of the threaded axial hole intended to receive the fixture bolt; if spanner flats are provided these are located adjacent the extremity of the head end of the anchor.
As stated above the prosthesis is usually secured to a transmucosal spacer member which in turn is secured to an implanted anchor. However the known systems are unsatisfactory in a number of respects. This is the case mainly because it is generally difficult to locate and dispose the implanted anchor so that it is not only adequately supported by regrown bone, but also may readily receive its intended attachments. In the case of a plurality of implanted inserts, the surgeon attempts to obtain parallelism for all the implanted anchors; this is difficult to achieve. Also the main requirement governing the procedure for initial implanting is that there should be caused minimal trauma, both to the bone and to other tissue etc; and this often mitigates against optimal location and disposition from other points of view. Here it should be mentioned that the shape and disposition of the eventual prosthesis is governed to a great extent by the requirements and physiology of the particular patient being treated. And in short, it is difficult to match optimal anchor location with the various other requirements.
Accordingly in the known implant systems and procedures, some degree of adjustability or adaptation has been provided so that the various and often conflicting requirments of oral bone anchor implant surgery can be met.
For example it has been proposed that the transmucosal spacer employed between the prosthesis and the anchor should be either bendable or otherwise deformable into the shape required, both as to its orientation in azimuth relative to the implanted anchor and as to the angle of inclination relative to the axis of the anchor of its region remote from the anchor; or the required shape is achieved in that a temporary transmucosal spacer is first inserted, then deformed into the shape required, and then removed and used as a model for a permanent transmucosal spacer. For example the deformed temporary transmucosal spacer may be used to make a mould for casting a permanent transmucosal spacer having inherently the final shape desired.
These procedures in turn involve disadvantages. If a permanent transmucosal spacer is deformed there is risk of undue stress being caused either to the bone, or to the implanted anchor, or to the transmucosal spacer itself. If the proceedure of using a temporary transmucosal spacer as a model is employed, there will be a delay in treatment which will adversely affect the patient.